Frequency comparison and control circuit



July 29, l947- f N. l. KORMAN FREQUENCYl COMPARISON AND CNTROL CIRCUIT 2sheets-sheet 2 Filed Oct. 18, 1944 l i EN Mfr/HMH K BY fsfirme/vifPatented July 29, 1947 FREQUENCY COMPARISON AND CONTROL CIRCUITNathaniel I. Korman, Camden, N. J., assigner to Radio Corporation ofAmerica, a corporation of Delaware Application Qctober 18, 1944, SerialNo. 559,189

(Cl. Z50- 36) M Claims. l

This application relates to a method of and means for comparing thefrequency of a source of wave energy the frequency of which may drift,with the frequency of wave energy of a substantially constant frequency,(standard source), and for producing current or energy which has acharacteristic that changes when the frequency relation of the comparedcurrents changes.

Such means are of wide use in the radio and allied arts. For example, itis often desirable to be able to check the operating frequency of asource of oscillations which may drift, and this application disclosesan improved and simplified method and means for accomplishing this.

For example, in receivers of the heterodyne type, the received wave orthe local oscillator may change in frequency so that the resulting I. F.frequency may not coincide with the tuning of the I. F. stages. Intiming modulation systems usually an oscillation generator the frequencyof which may be varied by the modulation is used. The oscillator mayalso drift in mean frequency of operation. In this application I show aniinproved, efficient and simplified means for comparing the frequency ofoscillations from sources of this type with oscillations ofsubstantially constant frequency to ascertain whether or-not thefrequency of the oscillations from the said sources are as intended.

This application also relates to methods of and means for ascertainingthe relation of the frequency of a source of oscillations with respectto a standard source frequency and for producing currents or energy acharacteristic of which depends on the relation of the frequency of thesource to the standard source.

This' application also relates to methods of and means for ascertainingthe relation of the frequencies of two sources one of which is of fixedfrequency, producing currents or energy characteristic of the deviationsand direction of deviation of the variable source frequency from thefixed source frequency and for stabilizing or retuning the source thefrequency of which may drift by means of the produced current or energy.

The broad general object of this invention is improved frequencycomparison and control. 1

More in detail, the object of this invention is simplified, improved andeilicient means for cornparison of the frequencies of two sources of`os. cillations one f which is of substantially fixed frequency, and onewhich is of a frequency which. may drift or vary slowly.

AV further object of this invention is means for comparing thefrequenciesof two sources as.

described in the preceding paragraph, and simplied, improved andefhcient means for deriving currents or energy characteristic of thefrequency relation of oscillations from the two sources.

An additional object of this invention is means as described in thepreceding paragraph, and simplified, improved and emcient means forusing the derived currents or energy to correct the frequency of theoscillations the frequency of which may vary.

Other objects and the manner in which the saine and the foregoingobjects are attained will appear from the description which follows.

In describing my invention in detail, reference will be made to theattached drawings wherein Fig. l shows schematically and by blockdiagram the essential features of a frequency comparison and controlcircuit arranged in accordance with my invention, by means of which myimproved method can be carried out. In Fig. 1, which is given merely byway of example, the oscillator the frequency of which may vary isdesignated as a frequency modulated oscillator of a frequency modulationsystem. The invention is equally applicable to oscillators used forother purposes, such as heterodyning oscillators of transmitters orreceivers.

Figs. 2 and 3 each illustrate a preferred embodiment of my improvedfrequency comparison and control means. In Figs. 2 and 3, as in Fig. 1,certain of the apparatus and means used is shown merely by rectangle,since the same may be conventional and per se forms no part of thepresent invention. Other features are shown by simplified circuitdiagram.

` Figs. 4to 7, inclusive, are curves used in describing the operation ofthe keyer in unit 5, the switching means in unit 3, the discriminator inunit 4, and the manner in which the frequencies of the two sources arecompared, the nature of the current produced by such comparison, and themanner in which the same is used to retune the oscillator in unit I.

As illustrated in Fig. 1, a timing modulated wave generator included inrectangle l is to be stabilized in operation so that its mean or averagefrequency is held substantially Constantin the assigned band. Aself-excited oscillator may be used here and may be modulated infrequency in any well known manner.

The unit 2 includes a source of oscillations of fixed frequency and, ifdesired, frequency multipliers to-provide a stable output at the pro-permean frequency of the modulated wave from,

'gigaset 3 unit 4. A crystal controlled oscillation generator ispreferred in unit 2.

The unit 3 includes a device for alternately feeding the outputs ofunits I and 2 to a discriminator and detector in unit 4. The device inunit 3 may be either mechanical or an electronic switch which is keyedby the output of a keyer in unit 5. The keyer in unit 5 maycomprise awave generator providing a wave to key the switcher in unit 3, or maycomprise mechanical means for providing keyed direct current at itsoutput.

The current out of 5 may be of sine wave form or rectangular. In oneexample illustrated, the apparatus in 5 is a multi-vibrator oscillatorof a well known type wherein sustained oscillations of square wave formsuch as illustrated in Fig. 5 are generated. These currents or voltageswhich may be below audibility or, if desired,

above audibility, are fed to the unit 3 which herein is referred to as aswitching stage. The

function of the unit 3 is to permit output from the modulated non-stableoscillator I, and output from the stable oscillation source 2 to be fedto the discriminator and detector in 4 alternatively for equal timeperiods set by the rate of recurrence of the current or oscillationsfrom unit 5.

The object of this apparatus is to compare the mean frequency of theoscillations of the modulated generator in I with the frequency of theoscillations from the reference source in 2.

Assume that the mean frequency out of unit I is as desired. Then it isequal to the frequency of the source in 2 and the frequency of theoscillations fed alternatively by mechanical or electronic switch stage3 to the discriminator in 4 are equal and say Fo and represented byastraight line in Fig. 4.. The output of a discriminator and detector inunit. 4 will then be constant (Fo Fig. 6) and represented by a straightline, say V in Fig. 7. If, however, the mean frequency of the modulatedoscillations out of unit I hasvaried and is say higher than thefrequency of the oscillations from source I, then as 3 is switched bythe output yof the keyer in unit the input to the discriminator in unit4 is first at frequency Fo, Fig. 4, and then at frequency Fm (modulatedoscillations out of unit I), Vand back at Fo.- If the drift of themodulated source in unit I is in the opposite direction when this sourceis switched through, the pulses Fm passed to unit 4 input are below thefrequency F0, as shown by dotted lines in Fig. 4.

Now the output of the discriminator and detector changes, as illustratedin Fig. 6, and there is an increased output from unit 4 Iwhen themodulated oscillations have a mean frequency Fm above the frequency ofthe source in unit 2, and the stage I is feeding through to the input ofthe unit 4. rlhe voltage at the detector output Varies from Vo to Vu,Fig. 7, as the stage 3 switches the input to the discriminator anddetector in unit 4. The wave form at the output of unit 4 is then asillustrated` by full lines in Fig. 7.

If the drift of the modulated source in unit I is in the oppositedirection and say through the frequency Fo of the reference source to afre- 'quency F'm, Figs. 4 and 6, then the output of the discriminatorand detector in unit 4 drops to say Vd, Fig. 4, when the switch in 3lets the output of the modulated oscillator I through. The wave form isthen as shown -by dotted lines and full lines in Fig. 7.

Thus from the discriminator and detector in unit 4 I derive a voltagewhich is keyed from a base value an amount depending on the drift of themean frequency of the modulated wave from unit I, and in a directiondepending upon the relation of this mean frequency to the frequency ofthe oscillations generated in unit 2. The discriminator and detector inunit 4 has a linear characteristic over the range of the frequencieswhich may be present at its input.

A filter circuit in unit 6 functions to remove the modulation present onthe output of the frequency modulated oscillator in unit I. The lter in6 is not needed in cases where the tuning device used to stabilize themean frequency of the modulated oscillator in unit I is not responsiveto this modulation.

At the output of the filter in 6 then is obtained a square wave whoseamplitude is proportional to the differences between the averagefrequency of the modulated oscillator in unitV I and the frequency ofthe stabilized 0r xed frequency output of unit 2. This square wave isplotted against time in Fig. 7, and is either in phase or out of phasewith the output of the keying means in 5, depending upon Whether themean frequency of the oscillations out of I is higher or lower than thefrequency of the stabilized oscillations out of unit 2, and on thephasing of the switcher in 3 and the discriminator characteristic in 4.For example, if Figs. 5 and 7 are compared, it will be seen that whenthe mean frequency of the oscillator unit I is above Fo the wave outputof the discriminator 4 and filter 'G shown in full lines in Fig. 7 is inphase with the keying voltage out of unit 5, shown in Fig. 5. When themean frequency of the oscillator unit I is below F0 then the keyingvoltage ouI'l of 5 and output of the discriminator 4, shown in dottedlines, Fig. 7, oppose.

It will also be apparent by reference to Fig. 6 that reversing thediscriminator characteristic so that it is as shown by dotted line Itherein instead of by full line 3, will reverse the output of thedetector in unit 4 and filter in unit E, as shown by dotted lines inFig. 7. Note that this output now opposes in phase the output from unit5, whereas the output of unit 4 shown in full lines is in phase with theoutput from unit 5. Obviously, by reversing the operation of themechanical or electrical switch in unit 3 the same result can beobtained. Also by reversing the operation of the keying voltage orcurrent out of the unit 5 will result in reversing the output of thediscriminator 4 in filter 6. Any of these expedients then may Ibe usedto provide out of 6 for use in the tuning apparatus in T a Voltage of apolarity such as to retune the oscillator in I in the proper directionto bring the mean frequency thereof in agreement with the frequency ofthe reference oscillations supplied from 2.

In any case, the Square wave output from the filter E is either in phase(full lines, Figs. 5 and 7) or out of phase (dotted lines, Fig. 7; fullline, Fig. 5), with the output of the keyer in 5 depending on whetheroscillator I is higher or lower in frequency than oscillator 2v, andthis phase relation is such as willbe seen in detail hereinafter to actthrough the unit I to properly tune the frequency modulated oscillatorinrl to restore equality between the frequencies fed through switcherstage 3tounit4.

The outputs of units 5 `and 6 are combined, in one of a number ofrways,to control the tuning of the oscillator in I to hold the same at thefre- 5 quency of the reference oscillations produced by oscillator 2.One such device for retuning the oscillator in I is a Variable capacitoror inductance in the oscillator circuit driven byv a 4direct cur-rentmotor whose eld and armature are actuated by the outputs of the filter 6and the keyer 5, respectively. The essential features of thisarrangement have been shown in Fig- 2 of the drawings. This Fig. 2 alsoillustrates an embodiment of my invention wherein the keyer in the u'nit5 andthe switchingmeans in the unit 3 are mechanical.

The oscillator in I may include an electron discharge device having atankY circuitT coupled with its anode and tuned by a motor drivencondenser. Additional capacity not shown may be includedV in the circuitT. The oscillator` maybe frequency modulated in any known ymanner inaccordanceA with control potentials. For example, reactance tube`modulation .as disclosed in Crosby patents No. 2,279,659, dated Apri-llll, 1942, and. No. 2,279,660, dated April 14, 1942, may be used. Themean frequency depends upon the tuning of T and the tuning of T dependsupon rotation of the motor in '1. Output from the unit I in tank circuitT is supplied Vby way of a mechanical switch S in unit 3 to the unit 4during a portion of a cycle of rotation of S, say half of a cycle of therotation of the switch S. Output from the unit 2, i. e., oscillations ofthe reference frequency are supplied through the switch S in unit 3 forthe remaining lpart of this cycle. The switch S is on .a shaft whichalso carries a swit-ch Si in the unit 5 and the switches S and S1 aresynchronized and may be drivenV by a motor 20 at say a sub orsuper-audible rate. The switch Si alternately applies direct currentpotentials of opposed polarity from the source 22 to the armaturewinding 28 of a. direct current motor 30 in unit 1 driving a condenserin the tank circuit T.

When the switch S and S1 are in the position shown high frequencycurrent flows from the oscillator inv I t-o the input of thediscriminator in 4., and direct current flows from the positive end ofsource 22 in one direction through the armature winding 28 of the motor30. When the switch motor 20 rotates the switches Si and S through ahalf cycle of rotation the input to the discriminat'or ll is coupled tothe output of the oscillator in 2 andthe winding 28 ofthe motor 30 iscoupled to the negative terminal of the source 22 soY that current flowsthrough this Winding in an opposite direction. Relative movement ofswitches S and Si through half a cycle in either direction will changethis relation so that the oscillations out of unit I are supplied to theinput of 4 when the negative terminal of source 22 supplies current tothe winding 28.

The output of the filter in B is supplied to the eld winding 32 of themotor 30. When the sources in units I and 2 are of like frequency theoutput of the discriminator and detector is constant, i. e., there is nopulse output since the voltage is V as illustrated in Fig. 7, thewinding 32 is not energized, and the rotor of the motor 30 is.stationary, so that the oscillation generator I operates at the samemean frequency as long as this condition is maintained. If the frequencyof'. the unstable oscillator in l drifts .below Fo so that there isa-pulse output from the discriminator and detector in 4 which has pulsepeaks below V0 to provide an output Va, Fig. 7, in phase opposition. tothe output of unit to winding 28, then the motor 3U is caused to operatein a direction such as to retune the tank circuit Tin a proper sense tobring the frequency of the oscillator generator I back to the frequencyof the oscillator generator 2. If the drift has been in the oppositedirection so that the frequency of the generator in I is above F0 thepulse peaks out of the detector are above V0 at say Vu and the polarityof the current through the winding 32 is reversed with respect to thepolarity of the current out of the unit 5 through winding 28, and thedirection of the rotation of the motor 30 reverses to .again tune T in asense such as to bring the mean frequency of the oscillations generatedin I to the frequency of the oscillations generated in 2,

In the embodiment of Fig. 3 the oscillation generator in unit I may beof the type wherein the tank circuit is associated with the outputimpedances of a pair of reactance tubes connected for modulation asdescribed say in U. S. Patent #2,279,661. The balanced combining andrectifying system in unit 'l provides a differential output which in onedirection increases the frequency of the oscillator in l and in theother direction decreases the frequency thereof.

The switching means in unit 3 may then coniprise a pair of electro-ndischarge devi-ces and i2 having their anodes tied together to Isupplyinput to the discriminator and detector in unit 4 from the unstableoscillator in I or the stable oscillator in 2. Tube 4l), for example,has its input electrodes including its control grid coupled to theoutput of unit I while tube 42 has its input electrodes including itscontrol grid coupled to the output of unit 2. The two tubes also includescreening electrodes by means of which they are switched alternativelyto pass and to block passage ofv excitation from unit I or 2 to unit 4.

The multi-vibrator tubes 50 and 52- in keyer 5 have `their anodesconnected through resista-hoes to a positive source of potential thenegative terminal of which `is grounded and thereby connected to. thecathodes of these two tubes, The control grids and anodes of the twotubes are cross-connected through coupling condensers and the anodes ofthe two tubes 523 and 52 are also connected to the screening gridelectrodes` of the switching tubes 4) and 42. The bias resistancesbetweenV the control grids and cathodes of the tubes 56 and 52 and thecondensers C and C in the couplings between the anodes and control gridsare such that the tubes 5l] and 52 generate in a well known mannersustained oscillations of square wave form and of a sub-audible andsuperaudible frequency. In generating these oscillations the `tubes arealternatively conductive-'so that the potential-s on the anodes thereofrise and fall at the oscillation rate and pass from a value which ispositive sufliciently to key tubes 40 and s2' to a conductive state to avalue which is sufiiciently less positive or negative to out these tubesoff. Thus in operation the tubes lll and t2 are alternatively keyed o-nand off by keying of the screen grid potentials thereof. Then thediscriminator circuit in l has as input oscillations from thesource I orfrom the source 2 alternatively present.

The anodes of the tubes 59 and 52 are also connected to the oppositeends of the winding 5S so that the generated oscillations are set up in'this winding to appear in the winding 58 and'to be applieddifferentially to the diode detectors D and DI. These diode detectorshave their cathodes coupled in opposition by resistances Iiland 62shunted by condensers of suicient size to remove any voltagesor currentchanges ofmodulation frequency to prevent. modulation of the frequencymodulated generator in I by such changes super-impressed on the controlpotentials. The output of the filter 6 is supplied to a secondtransformer 'III having its secondary winding connecting the diodes Dand DI in parallel. The diodes D and DI as connected compare and detect.the phase relation of theoutputs of units and 6.

Inspection of this circuit will show that the voltages impressed from 5Son the diodes D and DI are in phase and out of phase respectively, withthe voltages impressed in parallel on these diodes by transformer 10.Moreover, the phase relation of the output at 56 and the output of unit6 depends upon whether the mean frequency of the modulated oscillator ini is ahead or behind the frequency of the stabilized source in 2. Sincethe detector D and DI outputs which appear across resistances 6B and S2are differential the polarity of the output depends upon whether themean frequency of the source to be controlled in unit I is ahead orbehind the frequency of the standard source in unit 2. The term polarityas used here is not to mean that the cathode of one diode is negativewhen the cathode of the other diode is positive and vice Versa. The termdoes mean, however, that if we assume that when the sources in I and 2are in synchronism the cathodes of D and DI are say positive smallsubstantially equal amounts and the frequency of I then shifts thecathode of one diode say D will become more positive and the cathode ofDI will become less positive. If the shift in frequency of the source inI is in the opposite direction then the cathode of diode D would havebecome less positive and the cathode DI more positive.

The potentials developed at the diode cathodes are proportional to thedifference between the frequencies out of sources I and 2 and a meter 65shunting resista-noos 50 and 62 may be calibrated to show not only thisdifference but also give sense indications, i. e., denote Whether.source i is of higher or lower frequency than source 2.

The potential appearing at the cathode end of resistances 6B orresistance 62 may be used to retune the oscillation generator in I inany desired manner. For example, either of these potentials may beapplied as a control voltage to a single reactance tube associated withthe tank circuit T to tune the same and make the necessary corrections.In the embodiment illustrated the potentials at the diode cathodes areapplied differentially to two reactance tubes 'lil and 8l), described indetail hereinafter.

The phase relation of the voltages out of units 5 and 6 is made such, byone or the other of the means described hereinbefore, as to vary theeffective reactance of the tubes 'l0 and 80 connected with the generatorin I in a. sense to bring the mean frequency of the oscillator'l towardthe frequency of the oscillator 2. In addition to the means describedhereinbefore for properly relating the phase of the output 5 to thephase of the output of E to obtain frequency correction in the desiredsense, it will be noted that the direction of tuning can also bereversed by reversing the polarity of either of the windings oftransformers 56 and 78. The condensers 60 and B2 lter out the switchingfrequency components before they reach the reactance tubes and causeundesired modulations.

The reactance tube controlled oscillation generator in Fig. l may takemany forms. For eX- ample, I may use frequency modulated oscillationgenerators of the nature disclosed in many 8 prior art patents, such as,for example, Crosby U. S. Patent #2,279,659, 2,279,660, 2,279,661 and2,324,282.

In Fig. 3 I show a system substantiallyas disclosed in Fig. 2 of CrosbyU. S. Patent #2,279,661. The generator comprises a tube 64 having ananode control grid and cathode regeneratively coupled by a tank circuitT for the production of oscillatory energy of frequency determined inpart at least by the tuning of the tank circuit T. The generator has itsgrid supplied with direct current biasing potential by resistance R3 andits anode connected to a +B source. The negative terminal of this sourceand the cathode are grounded. The generator is coupled to the input ofthe unit 3, i. e., the switching stage, and has an output coupled asdesired to any utilization circuit.

The reactance tubes 'lil and 8D have their anodes tied together andconnected to the anode end of tank circuit T. The cathodes of thereactance tubes are tied together for radio frequency potential and tothe cathode of the oscillator tube 64 so that the reactive effectproduced in these tubes 1|] and 80 shunt a part at least of the tankcircuit* T. The phases of the radio frequency voltages on the anodes ofthese tubes are alike. The grid 14 of tube I0 obtains a radio frequencyvoltage shifted in phase about 90 by virtue of the action of phaseshifting resistance R and phase shifting condenser C. The grid 84 of thetube likewise is supplied a voltage shifted in phase about With-respectto the anode voltage of this tube by virtue of the action of phaseshifting resistance RI and inductance L. The condensers CC are merelycoupling condensers and of little or no eect on the phasing of thevoltages. Since tube I9 obtains its grid voltage by a capacitivereactive drop while the tube S0 obtains its grid 84 voltage from aninductive reactive drop the eiective variable reactance of the platecircuits of the two tubes are of opposite sign, one being predominantlycapacitive reactance, the other being predominantly inductive reactance.Since an increase in ampliiication decreases the effective inductance inthe plate circuit of say tube 80, and increases the effective capacityin the plate circuit vof tube 10, it can be seen that if theamplification of `both tubes vary'in the same direction the reactiveeffects tend to cancel. However, it can also be seen that ifdifferential control is applied as disclosed here, an increase in theamplification of one tube while decreasing the amplification of theother tube causes the reactive. effect to aid each other, thereby tuningthe tank circuit T. The control potentials developed across resistances6G and 62, which vary above and below about equal positive values asdescribed hereinbefore, are fed to the control grids 'I4 and 84 by wayofresistances 67 and 69 respectively. The grids 'I4 and 84 are biasednegatively the proper amount by source 15 connectedto the grids I4 and34 by resistances 1I and 8I` respectively. The source I5 and theresistances 61, 69, 7l and 8l are such that the differential potentialsfrom diodes D and DI change the respective tube 'Ill and 8D conductancesthe amount required to hold operation of the generator including tube 64and tank circuit T at the proper, and' substantially constant, meanfrequency.V

The modulation may be applied as desired as long as it is alsodifferential in its elect on the tube reactances. Ity may be applied tothe control grids'as shown. v f I In the sake of simplicity I have useda discriminator and detector system in Fig. 4 having a single slopingcharacteristic as illustrated in Fig. 6. Obviously in practice Icontemplate using a balanced discriminator circuit in order thatamplitude variations on the currents supplied to the unit Il will cancelout. For example, I may use a discriminator and detector of theoff-tuned circuit type referred to in the art as the Conrad typediscriminator. This discriminator may be substantially as disclosed inConrad U, S. Patent #2,151,747, dated March 28, 1939. A preferred formof circuit for use here is disclosed in Seeley U. S. Patent #2,121,103,dated June 2l, 1938, and in Crosby U. S. Patent #2,230,212, datedJanuary 28, 1941.

If a filter is necessary in unit 5 it may be of the low pass type wherea sub-audible keying rate is used in the unit l5 o-f Fig. 2, or asub-audible multi-vibrator generator is used in unit y5 of Fig. 3. Thenfrequencies falling below the modulation band are passed. In case asuper-audible rate of keying is used in Fig. 2 or a super-audiblegenerator is used at -5 in Fig. 3, a high pass filter passingfrequencies above the modulation frequencies may be used in unit 6.

I claim:

1. In a signalling system two sources of oscillations the frequency ofone of which may drift with respect to the frequency of the other,reactance tube control means associated with said one source to controlthe frequency thereof, a discriminator and detector circuit, a switchingcircuit for alternatively connecting the outputs from said sources tosaid discriminator circuit at a selected rate to produce pulses of saidrate when said one source drifts, of a phase depending on the directionof drift of the frequency of said one source, means for producingvoltages of a fre quency equal to said selected rate, means for comubining said produced voltage with the output of said discriminator anddetector circuit to obtain voltages of an amplitude determined by theextent of drift of the frequency of said one source, and means forcontrolling' said tube reactance in accordance with said last mentionedvoltages.

2. The method of comparing the frequency of a source of oscillations ofunstable frequency with oscillations from a source of substantially thesame and fixed frequency which includes these steps, producing currentpulses recurring at a substantially constant rate, subjecting pulses ofsaid oscillations of unstable frequency, recurring substantially at saidconstant rate, and alternatively occurring pulses of said oscillationsof fixed frequency to a frequency demodulation action, to derive pulsesthe phase of which depends on the frequency of the unstabilizedoscillations and pulses of Xed frequency, and combining said producedcurrent pulses with said derived pulses to provide resultant potentialswhich vary from a base value in a direction depending on the frequencyrelation of the unstable oscillations and stable oscillations.

3. The method of comparing the frequency of a source ofr oscillations ofunstable frequency with oscillations of substantially the same and fixedfrequency which includes these steps, producing current pulses recurringat a substantially constant rate, subjecting pulses of said twooscillations recurring alternately at the same substantially constantrate to a timing demodulation action to derive pulses of current, oflike pulse rate and timing, the phase of which depends on Whether thefrequency of the unstable oscillations is above or below the frequencyof the oscillations of fixed frequency, and combining said producedcurrent pulses with said derived pulses of current to provide resultantpotentials which vary from a base value in a direction depending on thefrequency relation of the unstable oscillations and stable oscillations.

4. The method of stabilizing 4the frequency of a source of oscillationsof otherwise unstable frequency by means of oscillations ofsubstantially the same and fixed frequency which includes these steps,producing current pulses recurring at a substantially constant rate,subjecting pulses of said two oscillations recurring alternately at thesame substantially constant rate to a frequency discriminating anddetecting action to derive pulses of current, of like pulse rate andtiming, the phase of which depends on whether the frequency of theunstable oscillations is above or below the frequency of theoscillations of fixed frequency, combining said produced current pulseswith said derived pulses of current to lirovide resultant potentialswhich vary from a base value in a direction depending on the frequencyrelation of the unstable oscillations and stable oscillations andcontrolling the frequency of said oscillations of unstable frequency inaccordance with said resultant potentials.

5. The method of comparing the frequency of a source of oscillations ofunstable frequency with oscillations from a source of substantially thesame and fixed frequency which includes these steps, generating currentpulses of substantially square wave form and of substantially constantfrequency, alternately selecting pulses of oscillations from said twosources at a pulse rate equal to the pulse rate of said generated pulsesand in synchronism therewith, detecting the selected pulses ofoscillations to derive pulses of current, of like pulse rateand timing,the phase of which depends on whether the frequency of the unstableoscillations is above or below the frequency of the oscillations offixed frequency, and combining said generated current pulses with `saidderived pulses of current to provide resultant potentials which varyfrom a base value in a direction depending on the frequency relation ofthe unstable oscillations and stable oscillations.

6. The method of stabilizing the frequency of a source of oscillationsof unstable frequency by means of oscillations from a source ofsubstantially the same and fixed frequency which includes these steps,generating current pulses of substantially square wave form and ofsubstantially constant frequency, alternately selecting pulses ofoscillations from said two sources at a pulse rate equal to the pulserate of said generated pulses and in synchronism therewith, subjectingthe selected pulses of oscillations `to a timing demodulation action toderive pulses of current, of like pulse rate and timing, the phase ofwhich depends on whether the frequency of the unstable oscillations isabove or below the frequency of the oscillations of fixed frequency,combining said generated current pulses with said derived pulses ofcurrent to provide resultant potentials which vary from a base value ina direction depending on the frequency relation of the unstableoscillations and stable oscillations,

and controlling the frequency of said oscillations of unstable frequencyin accordance with the resultant potentials.

7. In apparatus for comparing the frequencies of oscillations from twosources the frequency of one of which may drift with respect to thefrequency of the other, a discriminator and detector circuit, aswitching circuit for alternatively connecting the outputs from saidsources to said discriminator circuit at a selected rate to producepulses of said rate when said one source drifts, of a phase depending onthe direction of drift of the frequency of said one source, means forproducing voltages of a frequency equal to said selected rate, and meansfor combining said produced voltage with the output of saiddiscriminator and detector circuit to obtain voltages of an amplitudedetermined by the extent of drift of the frequency of said one source.

8. In frequency control apparatus two sources of oscillations thefrequency of one of which may drift with respect to the frequency of theother, a discriminator and detector circuit, a switching circuit foralternatively connecting the outputs from said sources to saiddiscriminator circuit at a selected rate to produce pulses of said ratewhen said one source drifts, of a phase depending on the direction ofdrift of the frequency of said one source, means for producing voltagesof a frequency equal to said selected rate, means for combining saidproduced voltage with the output of said discriminator and detectorcircuit to obtain voltages of an amplitude determined 'by the extent ofdrift of the frequency of said one source, and means for retuning saidone source of oscillations in accordance with said last mentionedvoltages.

9. In apparatus for comparing the frequencies of oscillations from twosources the frequency of one of which may drift with respect to thefrequency ofthe other, a frequency discriminator and detector circuit, aswitching circuit for alternatively and at a selected rate connectingthe said sources to said discriminator circuit to alternatively rectifyoscillations from said sources to supply rectified output, means forproducing output voltages of said selected rate, and a balancedrectifier system excited differentially by one of said outputs, andcophasially by the other of said outputs.

10. In apparatus for comparing the frequencies of oscillations from twosources the frequency of one of which may drift with respect to thefrequency of the other, a frequency discriminator and detector circuit,a switching circuit for alternatively and at a selected rate connectingthe outputs from said sources to said discriminator circuit toalternatively rectify the outputs Of said sources to supply rectifiedoutput, means for producing voltages of said selected rate, and abalanced rectifier system excited differentially by the output of saiddiscriminator and detector circuit, and cophasially by said producedvoltages.

11. In apparatus for stabilizing the frequency of oscillations from anunstable generator, a generator of substantially the same and fixedfrequency, a frequency discriminator and detector circuit, a switchingcircuit interposed rbetween said generators and said discriminator foralternatively connecting said generators to said discriminator circuitto alternatively rectify the oscillations to supply rectified output, apulse generator having two outputs, one of which is coupled to saidswitching circuitto operate the same, a balanced rectifier systemexcited by the output of said discriminator and detector circuit, and bythe generated pulses, and a tuning control circuit connecting 'saidbalanced rectier system to said unstable generator. l

12. In apparatus for comparing the frequencies of oscillations from twosources the frequency of one of which may drift with respect to the frequency of the other, a discriminator and detector circuit, a switchingcircuit for alternatively connecting the outputs from said sources tosaid discriminator circuit at a selected rate to produce current pulsesof said rate when said one source drifts, of a polarity depending on thedirection of drift of the frequency of said one source, switching meansfor producing direct current voltages of a frequency equal to saidselected rate, and 'a motor having two windings, one of which is coupledwith said discriminator and detector and the other of which is coupledwith said last mentioned means.

13. In frequency control apparatus, two sources of oscillatory energythe frequency of one of which may drift with respect to the frequency ofthe other, a discriminator and detector circuit, a switching circuit foralternatively connecting the output from said sources to saiddiscriminator circuit at a selected rate to produce pulses of said ratewhen said source drifts, of a phase depending on the direction of driftof the frequency of said one source, a motor having two windings, meansfor producing voltages of a frequency equal to said selected rate andsupplying the same to one motor winding, a coupling between saiddetector circuit and the other winding of said motor, and reactive meansoperated by said motor for controlling the frequency of the oscillatoryenergy of said one of said two first sources.

14. In apparatus for comparing the frequencies of oscillations from twosources, the frequency of one of which may drift with respect to thefrequency of the other, a frequency discriminator and detector circuit,a switching circuit for alternatively and at a selected rate connectingthe said sources to said discriminator circuit to a1- ternativelyrectify oscillations from said sources to supply rectified output, of apolarity depending on the relative frequencies of said two sources, amotor having two windings, means for producing direct current potentialsrecurring at said selected rate and applying the same to one of saidmotor windings, and a connection between the output of said detector andthe other of said motor windings, the arrangement being such that thedirection of rotation of said motor depends upon the polarity of saidrectied output.

NATHANIEL I. KORMAN.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,647,270 Burton Nov. 1, 19272,151,127 Logan et al. Mar. 21, 1939 2,126,910 Moseley Aug. 16, 19382,111,598 Marrison Mar. 22, 1938

